28-03-2011, 02:40 PM
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1. INTRODUCTION
The main function of a power system is to supply electrical energy to its customers with an acceptable degree of reliability and quality. Among many other things, the reliability of a power system depends on trouble free transformer operation. Now, in the electricity utilities around the world, a significant number of power transformers are operating beyond their design life. Most of these transformers are operating without evidence of distress. In Power Link Queensland (PLQ), 25% of the power transformers were more than 25 years old in 1991. So priority attention should be directed to research into improved diagnostic techniques for determining the condition of the insulation in aged transformers.
The insulation system in a power transformer consists of cellulosic materials (paper, pressboard and transformerboard) and processed mineral oil. The cellulosic materials and oil insulation used in transformer degrade with time. The degradation depends on thermal, oxidative, hydrolytic, electrical and mechanical conditions which the transformer experienced during its lifetime.
The condition of the paper and pressboard insulation has been monitored by (a) bulk measurements (dissolved gas analysis (DGA) insulation resistance (IR), tanö and furans and (b) measurements on samples removed from the transformer (degree of polymerization (DP) tensile strength). At the interface between the paper and oil in the transformer, interfacial polarization may occur, resulting in an increase in the loss tangent and dielectric loss. A DC method was developed for measuring the interfacial polarization spectrum for the determination of insulation condition in aged transformers.
This paper makes contributions to the determination of the insulation condition of transformers by bulk measurements and measurements on samples removed from the transformer.
In this research project, thorough investigations were also undertaken of the conventional electrical properties, along with interfacial polarization parameters of the cellulosic insulation materials. The interfacial phenomena are strongly influenced by insulation degradation products, such as polar functionalities, water etc. The condition of the dielectric and its degradation due to ageing can be monitored by studying the rate and process of polarization and can be studied using a DC field. Furthermore, this is a non-destructive diagnostic test.
A retired power transformer (25 MVA, l1/132 kV) and several distribution transformers were used for the experimental work. The results from these transformers will be presented and an attempt will be made to correlate the electrical and chemical test results. The variation of the results through the different locations in a power transformer will be discussed with reference to their thermal stress distribution.
2. EXPERIMENTAL TECHNIQUES
Experimental techniques used for the assessment of insulation condition in aged transformers are described in the following section.
2.1 Conventional Electric Tests
The dissipation factor and capacitance were measured at 50 Hz using a Schering bridge. Power frequency breakdown strength was measured by using the step by step method. The standard wave shape of l. was used for determining the negative lightning impulse breakdown strength.
2.2 Interfacial Polarization Spectra (IPS) Measurements
When a direct voltage is applied to a dielectric for a long period of time, and it is then short circuited for a short period, after opening the short circuit, the charge bounded by the polarization will turn into free charges i.e, a voltage will build up between the electrodes on the dielectric. This phenomena is called the return voltage. After applying the field for a time t, the polarization is expressed by P(t) = P0 F(t), where P0 = αE is the steady state value of the polarization, α is a proportionality factor between the polarization and the field strength (E), called the polarizabiity, F(t) is the relaxation function of the polarization describing the development of polarization in time and P is the bound charge density.
Polarizabiity will increase when polarization increases. So the maximum return voltage can be correlated with the polarizability of the material.
With the development of polarization, the charge bounded on the electrodes tends to grow. In the external circuit maintaining the field, this growth will cause an absorption current given by Ja(t) = P(t) = d/dt P (t). With polarization approaching a steady state value, the current decays in time to zero. As for polarization, the absorption current is proportional to the field strength. So the initial value can be written as Ja (0) = βE, where β is the proportionality factor between absorption cur rent and field strength, and is called polarization conductivity. It can be shown that the initial slope of the return voltage is proportional to the polarization conductivity. When the return voltage approaches its maximum value quickly, the initial slope of the return voltage is larger. Another parameter termed as ‘central time constant’, i.e. the time at which the return voltage is maximum, is also dependent on the polarization conductivity. Hence the fundamental characteristics of the dielectric can be measured by return voltage measurements.
An experimental set up with an IBM PC and a programmable electrometer was developed and implemented to measure the return voltage of a two terminal dielectric system. The charging voltage was 100 volt DC for the retired transformer insulation samples. The developed software was used to control the electrometer. Adsorbed moisture and temperature of the oil-paper insulation adversely affects the return voltage measurement. So the return voltage measurement was always conducted at a known and low oil-paper moisture content and at ambient environmental conditions (20 — 25° C).
A typical return voltage wave shape of a specimen from the retired transformer is shown in Fig. 1. The relevant parameters (maximum return voltage, initial slope and central time constant) are identified in Fig. 1. Initial slope is the slope of the return voltage graph (with linear approximation) for first few seconds. As interfacial polarization is predominant at longer time constants, the spectrum of the return voltage was investigated by changing the charging and discharging time over a range of times greater than 1 second until the peak value of the maximum return voltage was obtained. The ratio of charging and discharging time was two. Then the spectra of maximum return voltage and initial slope were plotted versus the central time constant (the time at which the return voltage is maximum). The peak value of the maximum return voltage (from the return voltage spectrum) and the corresponding initial slope (from the initial slope spectrum), along With central time constant (from either of the spectrum), are the parameters used to assess the insulation condition from the return voltage measurements.
2.3 GPC Analysis
Gel permeation chromatography provides a detailed molecular weight distribution of the polymer. GPC is a chromatographic technique which uses highly porous, non-ionic gel beads for the separation of polydispersed polymers in solution. GPC separates polymer molecules on the basis of their hydrodynamic volume. Cellulose is not soluble in any common GPC solvents. Hence, for GPC measurements the cellulosic materials had to be derivatized to enhance their solubility in these solvents. For this purpose, a cellulose tricarbanilate derivative was prepared.
The molecular weight distribution of the cellulose tricarbanilate was measured using a Waters Chromatograph equipped with a variable wavelength tunable absorbance detector. Four ultrastyragel columns were used in series in the Chromatograph, with tetrahydrofuran (THF) as the eluent. The elution profile was acquired by interfacing to an IBM computer.
3. RESULTS AND DISCUSSIONS
Paper wrapped insulated conductor specimens 200 mm long and pressboard samples of dimension 80*80 mm were collected from an aged power transformer. Several distribution transformers were also tested.
3.1 Case Study 1: Kareeya Transformer
A 25 year old, 25 MVA, 132/11 kV transformer from Kareeya power station, was used to investigate the quality of the insulation using electrical and chemical testing techniques. Since the aged transformer had been exposed to air after dismantling, the samples had to be processed. The moisture content of processed samples varied in the range 0.5 to 1.3%.
To examine the differences that exist between the high stress and low stress insulation samples, the samples were collected from top, middle and bottom coils of low voltage and high voltage windings of the transformer. The schematic diagram of a low voltage winding is shown
There were 90 coils/phase and 18 turns or layers of conductor/coil in the low voltage windings. There were 60 coils/phase and 19 turns or layers of conductors/coil in the high voltage winding. The HV and LV conductors were of rectangular cross section 13.9 and 12 mm wide respectively and 2.6 mm thick with rounded corners. The test specimens for insulated conductor samples were made up by placing two samples side by side in a Perspex assembly, so that they overlapped each other for a length of 100 mm. With two insulated conductors placed side by side to form the specimen, the thickness of paper insulation between them was 1.0 mm and 0.8 mm for the HV and LV specimens respectively. Pressboard (of 0.2 mm thickness) samples were collected from the main bulk insulation between the high voltage and low voltage winding is shown
1. INTRODUCTION
The main function of a power system is to supply electrical energy to its customers with an acceptable degree of reliability and quality. Among many other things, the reliability of a power system depends on trouble free transformer operation. Now, in the electricity utilities around the world, a significant number of power transformers are operating beyond their design life. Most of these transformers are operating without evidence of distress. In Power Link Queensland (PLQ), 25% of the power transformers were more than 25 years old in 1991. So priority attention should be directed to research into improved diagnostic techniques for determining the condition of the insulation in aged transformers.
The insulation system in a power transformer consists of cellulosic materials (paper, pressboard and transformerboard) and processed mineral oil. The cellulosic materials and oil insulation used in transformer degrade with time. The degradation depends on thermal, oxidative, hydrolytic, electrical and mechanical conditions which the transformer experienced during its lifetime.
The condition of the paper and pressboard insulation has been monitored by (a) bulk measurements (dissolved gas analysis (DGA) insulation resistance (IR), tanö and furans and (b) measurements on samples removed from the transformer (degree of polymerization (DP) tensile strength). At the interface between the paper and oil in the transformer, interfacial polarization may occur, resulting in an increase in the loss tangent and dielectric loss. A DC method was developed for measuring the interfacial polarization spectrum for the determination of insulation condition in aged transformers.
This paper makes contributions to the determination of the insulation condition of transformers by bulk measurements and measurements on samples removed from the transformer.
In this research project, thorough investigations were also undertaken of the conventional electrical properties, along with interfacial polarization parameters of the cellulosic insulation materials. The interfacial phenomena are strongly influenced by insulation degradation products, such as polar functionalities, water etc. The condition of the dielectric and its degradation due to ageing can be monitored by studying the rate and process of polarization and can be studied using a DC field. Furthermore, this is a non-destructive diagnostic test.
A retired power transformer (25 MVA, l1/132 kV) and several distribution transformers were used for the experimental work. The results from these transformers will be presented and an attempt will be made to correlate the electrical and chemical test results. The variation of the results through the different locations in a power transformer will be discussed with reference to their thermal stress distribution.
2. EXPERIMENTAL TECHNIQUES
Experimental techniques used for the assessment of insulation condition in aged transformers are described in the following section.
2.1 Conventional Electric Tests
The dissipation factor and capacitance were measured at 50 Hz using a Schering bridge. Power frequency breakdown strength was measured by using the step by step method. The standard wave shape of l. was used for determining the negative lightning impulse breakdown strength.
2.2 Interfacial Polarization Spectra (IPS) Measurements
When a direct voltage is applied to a dielectric for a long period of time, and it is then short circuited for a short period, after opening the short circuit, the charge bounded by the polarization will turn into free charges i.e, a voltage will build up between the electrodes on the dielectric. This phenomena is called the return voltage. After applying the field for a time t, the polarization is expressed by P(t) = P0 F(t), where P0 = αE is the steady state value of the polarization, α is a proportionality factor between the polarization and the field strength (E), called the polarizabiity, F(t) is the relaxation function of the polarization describing the development of polarization in time and P is the bound charge density.
Polarizabiity will increase when polarization increases. So the maximum return voltage can be correlated with the polarizability of the material.
With the development of polarization, the charge bounded on the electrodes tends to grow. In the external circuit maintaining the field, this growth will cause an absorption current given by Ja(t) = P(t) = d/dt P (t). With polarization approaching a steady state value, the current decays in time to zero. As for polarization, the absorption current is proportional to the field strength. So the initial value can be written as Ja (0) = βE, where β is the proportionality factor between absorption cur rent and field strength, and is called polarization conductivity. It can be shown that the initial slope of the return voltage is proportional to the polarization conductivity. When the return voltage approaches its maximum value quickly, the initial slope of the return voltage is larger. Another parameter termed as ‘central time constant’, i.e. the time at which the return voltage is maximum, is also dependent on the polarization conductivity. Hence the fundamental characteristics of the dielectric can be measured by return voltage measurements.
An experimental set up with an IBM PC and a programmable electrometer was developed and implemented to measure the return voltage of a two terminal dielectric system. The charging voltage was 100 volt DC for the retired transformer insulation samples. The developed software was used to control the electrometer. Adsorbed moisture and temperature of the oil-paper insulation adversely affects the return voltage measurement. So the return voltage measurement was always conducted at a known and low oil-paper moisture content and at ambient environmental conditions (20 — 25° C).
A typical return voltage wave shape of a specimen from the retired transformer is shown in Fig. 1. The relevant parameters (maximum return voltage, initial slope and central time constant) are identified in Fig. 1. Initial slope is the slope of the return voltage graph (with linear approximation) for first few seconds. As interfacial polarization is predominant at longer time constants, the spectrum of the return voltage was investigated by changing the charging and discharging time over a range of times greater than 1 second until the peak value of the maximum return voltage was obtained. The ratio of charging and discharging time was two. Then the spectra of maximum return voltage and initial slope were plotted versus the central time constant (the time at which the return voltage is maximum). The peak value of the maximum return voltage (from the return voltage spectrum) and the corresponding initial slope (from the initial slope spectrum), along With central time constant (from either of the spectrum), are the parameters used to assess the insulation condition from the return voltage measurements.
2.3 GPC Analysis
Gel permeation chromatography provides a detailed molecular weight distribution of the polymer. GPC is a chromatographic technique which uses highly porous, non-ionic gel beads for the separation of polydispersed polymers in solution. GPC separates polymer molecules on the basis of their hydrodynamic volume. Cellulose is not soluble in any common GPC solvents. Hence, for GPC measurements the cellulosic materials had to be derivatized to enhance their solubility in these solvents. For this purpose, a cellulose tricarbanilate derivative was prepared.
The molecular weight distribution of the cellulose tricarbanilate was measured using a Waters Chromatograph equipped with a variable wavelength tunable absorbance detector. Four ultrastyragel columns were used in series in the Chromatograph, with tetrahydrofuran (THF) as the eluent. The elution profile was acquired by interfacing to an IBM computer.
3. RESULTS AND DISCUSSIONS
Paper wrapped insulated conductor specimens 200 mm long and pressboard samples of dimension 80*80 mm were collected from an aged power transformer. Several distribution transformers were also tested.
3.1 Case Study 1: Kareeya Transformer
A 25 year old, 25 MVA, 132/11 kV transformer from Kareeya power station, was used to investigate the quality of the insulation using electrical and chemical testing techniques. Since the aged transformer had been exposed to air after dismantling, the samples had to be processed. The moisture content of processed samples varied in the range 0.5 to 1.3%.
To examine the differences that exist between the high stress and low stress insulation samples, the samples were collected from top, middle and bottom coils of low voltage and high voltage windings of the transformer. The schematic diagram of a low voltage winding is shown
There were 90 coils/phase and 18 turns or layers of conductor/coil in the low voltage windings. There were 60 coils/phase and 19 turns or layers of conductors/coil in the high voltage winding. The HV and LV conductors were of rectangular cross section 13.9 and 12 mm wide respectively and 2.6 mm thick with rounded corners. The test specimens for insulated conductor samples were made up by placing two samples side by side in a Perspex assembly, so that they overlapped each other for a length of 100 mm. With two insulated conductors placed side by side to form the specimen, the thickness of paper insulation between them was 1.0 mm and 0.8 mm for the HV and LV specimens respectively. Pressboard (of 0.2 mm thickness) samples were collected from the main bulk insulation between the high voltage and low voltage winding is shown
